The present invention relates to an oscillator circuit adapted for use in a voltage controlled oscillator (VCO) of a PLL circuit or a carrier signal oscillator of a PWM demodulator.
Shown in FIG. 1 is an example of a conventional voltage controlled oscillator circuit. This oscillator circuit operates in the following manner. When a transistor Q.sub.3 is in the non-conductive state, a capacitor C is charged by a current i flowing from a constant current source I via a diode D.sub.2. At the instant when the charged voltage of the capacitor C exceeds an inversion threshold voltage of a Schmitt trigger circuit including transistors Q.sub.4 -Q.sub.6, the transistor Q.sub.6 is rendered non-conductive, the resistance between the collector and the emitter of the transistor Q.sub.7 decreases, a transistor Q.sub.8 forming a differential amplifier circuit together with a transistor Q.sub.9 is rendered non-conductive, the counterpart transistor Q.sub.9 is rendered conductive, and then the transistor Q.sub.3 is rendered conductive. In this manner, the state of the circuit is inverted as a result of which a current mirror circuit including transistor Q.sub.1 and Q.sub.2 operates so that the current i from the constant current source I flows into the transistor Q.sub.1 and the identical amount of current i is sunk by the transistor Q.sub.2 from the capacitor C. As a result of the discharge from the capacitor C, when the voltage across the capacitor C falls below a reset threshold voltage of the Schmitt trigger circuit to reset the latter, the transistor Q.sub.6 is rendered conductive, the resistance across the collector and the emitter of the transistor Q.sub.7 increases, the transistor Q.sub.8 is rendered conductive, the transistor Q.sub.9 is rendered non-conductive, and the transistor Q.sub.3 is rendered non-conductive. In this manner, the circuit returns to the initial state so that the current mirror circuit becomes inoperable and the capacitor C is again charged.
Accordingly, a triangular waveform voltage V.sub.A is developed at a terminal A to which the voltage of the capacitor C is outputted. A rectangular waveform voltage V.sub.B of identical duration (identical frequency) to that of the triangular waveform voltage V.sub.A is produced at the emitter of the transistor Q.sub.7. The frequency of the rectangular waveform voltage V.sub.B is proportional to the amount of the current i. In FIG. 2, there are shown waveform diagrams in which V.sub.S1 and V.sub.S2 represent the inversion threshold voltage of the Schmitt trigger circuit and the reset threshold voltage thereof, respectively, and +V.sub.cc and -V.sub.cc are power supply voltages.
The Schmitt trigger circuit of the oscillator circuit shown in FIG. 1 operates, with the utilization of the positive feedback of the transistors Q.sub.5 and Q.sub.6, with the transistors thereof in their saturated regions. Accordingly, the following disadvantages are encountered. Specifically, in the case where a rectangular waveform pulse V.sub.IN as shown in FIG. 3A is applied as an input voltage to the transistor, the collector current I.sub.c involves a minority carrier accumulation time t.sub.s as shown in FIG. 3B. In FIG. 3B, t.sub.r and t.sub.f are representative of a rise time and a fall time, respectively. Due to the minority carrier accumulation time, the operation of the transistor is delayed and high speed operation cannot be accomplished. For this reason, the circuit of FIG. 1 is unable to oscillate at a high frequency. In other words, the oscillator circuit shown therein has limited application as a high speed oscillator.
In view of the foregoing, an object of the invention is to provide an oscillator circuit in which the transistors used in the various parts of the circuit are operated in non-saturated regions so as to eliminate the minority carrier accumulation time thereby greatly improving the operational speed of the circuit.